In the manufacture of certain thin film devices, a substrate of each device is provided with a conductive "glue" layer, which comprises a film of palladium and which is applied to the substrate in a sputtering operation to facilitate the adhesion of a subsequently plated gold film to the substrate. In the plating of the gold films on the substrates, the substrates are mounted in a device known as a box plater with the palladium films (cathodes) in opposed relationship to respective anode plates also mounted in the box plater. To facilitate mounting of the substrates and the anode plates into the box plater through the bottom of the plater, and removal of the substrates and the anode plates from the plater through the bottom thereof, only upper ends of the palladium films and the anode plates make electrical contact with respective electrical contact pins for providing plating current to the substrates and the anode plates during a plating operation. The box plater, including the substrates and the anode plates, then is pre-immersed in an electroplating bath and a plating current is applied to the palladium films through the anodes and the plating bath to form the gold films on the palladium films.
However, when palladium-coated substrates are plated with gold in this manner, it has been found that the thickness of the gold film plated onto each substrate is significiantly greater adjacent the upper end of the substrate in comparison to the thickness of the gold film adjacent the lower end of the substrate. This is undesirable for various reasons, including the necessity for plating an excessive amount of gold on the substrates adjacent their upper ends in order to insure that a minimum required thickness of gold is present on the substrates adjacent their lower ends. As a result, because of the present high cost of gold, the manufacturing cost of the thin film devices is significantly increased.
Applicant has observed that the above-described gradient in the gold films on the substrates apparently is a result of the gold tending to plate onto the substrates initially in the vicinity of the contact pins in contact with the upper ends of the substrates, and then to plate progressively downward on the substrates to the lower ends thereof. This is attributed, at least in part, to the fact that because the palladium films are electrically contacted only adjacent their upper ends, and because palladium has a high electrical resistivity, in comparison to gold, the current densities in the plating bath initially are highest adjacent the contact pins at the upper ends of the substrates. As a result, gold initially plates onto each substrate adjacent the contact pins, and then progressively downward on the substrate, to form an initial thickness gradient in the plated gold which is retained thereby as additional gold is then plated onto the substrate.
Accordingly, a primary purpose of this invention is to provide a new and improved plating method and apparatus in which an electrically conductive material is plated essentially uniformly over a portion of an article being plated, particularly where the article portion has a high electrical resistivity, so as to reduce the amount of material which must be plated on the article in order to achieve a minimum preselected thickness at each point on the article.